linux-kernel - Re: [PATCH 2/2] iio: adc: Allow setting Shunt Voltage PGA gain and Bus Voltage range

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Date:   Sat, 29 Apr 2017 22:37:27 +0200
From:   Stefan Bruens <stefan.bruens@...h-aachen.de>
To:     Jonathan Cameron <jic23@...nel.org>
CC:     <linux-iio@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Hartmut Knaack" <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        "Peter Meerwald-Stadler" <pmeerw@...erw.net>,
        Marc Titinger <mtitinger@...libre.com>
Subject: Re: [PATCH 2/2] iio: adc: Allow setting Shunt Voltage PGA gain and Bus Voltage range

On Mittwoch, 26. April 2017 08:59:47 CEST Jonathan Cameron wrote:
> On 26/04/17 07:19, Jonathan Cameron wrote:
> > On 17/04/17 23:08, Stefan Bruens wrote:
> >> On Freitag, 14. April 2017 17:12:03 CEST Jonathan Cameron wrote:
[...]
> > 
> >> 4. Any user of the gain settings had to be made aware of the possibility
> >> to
> >> change it, no matter how it is exposed. Making it part of the scale, and
> >> thus changing the meaning of the raw values, would be breaking the
> >> existing ABI.> 
> > The raw values should indeed not change.  That was a missunderstanding on
> > my part.  Usually when a device has a PGA it is not compensated for in
> > the output. So normally it's up to the driver to 'apply' the effective
> > gain to the incoming reading.  When that isn't the case, it can be
> > considered some sort of internal trim - hence the use of calibscale for
> > this case.
> Mulling this over, calibscale might not work either in this case. The
> datasheet helpfully sometimes uses ranges and sometimes uses scale factors.
> There is also obviously the calibration register kicking around which would
> also be handled with calibscale if exposed to userspace (currently it isn't)
> 
> I'm out of time tonight so will think it bit more about this and get back to
> you in the next few days...

hardwaregain may be a viable option. For the shunt voltage, available values 
would be [0.125, 0.25, 0.5, 1.0], for the bus range we would have either [0.5, 
1.0] or [1.0, 2.0] for bus ranges [32V, 16V].

Does hardwaregain have the right semantics for shunt voltage gain and/or bus 
range?

Kind regards,

Stefan

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